1. Field of the Invention
The present invention relates to a color conversion definition correction apparatus for correcting a color conversion definition defining a relation between first color data representative of a color by coordinates on a first color space and second color data representative of a color by coordinates on a second color space in such a manner that a plurality of first color data has a one-to-one correspondence with a plurality of second color data, and a color conversion definition correction program storage medium storing a color conversion definition correction program which causes a computer to operate as such a color conversion definition correction apparatus.
2. Description of the Related Art
Hitherto, it is performed that image data is obtained in such a manner that an input device such as a color scanner and an electronic still camera is used to read an image (here, referred to as an original image), and reproduction image is obtained in such a manner that an output device such as a printing machine, a printer and a display is used to reproduce the original image in accordance with the image data thus obtained. In this case, there are determined a color conversion definition (an input profile) associating a color on the original image with a color representation on the image data, according to the input device, and a color conversion definition (an output profile) associating the color representation on the image data with a color on the reproduction image, according to the output device such as a printing machine and a printer, and the image data obtained by the input device through the original image is converted into image data adapted for the output device in accordance with a combined color conversion definition (a device link profile) of both the color conversion definitions (the input profile and the output profile), so that a reproduction image is outputted in the image data for the output device. In this manner, it is possible to obtain a reproduction image greatly close to the original image in color.
This is applicable between output device-to-output device too. Next, examples will be explained.
Hitherto, when a printing machine is used to perform a color image printing, it is performed prior to the color image printing that a color printer and the like is used to create a proof image which is extremely similar in color to an image to be printed in the printing machine. When the color printer and the like is used to create the proof image, the proof image is created in such a manner that detected are a color conversion definition (a printing profile) defining an association of color data representative of dot % of for example, C (cyan), M (magenta), Y (yellow), and K (black), for a printing machine of interest, with a color of the actual printed matter outputted in accordance with the color data, and a color conversion definition (a printer profile) defining an association of color data representative of dot % of for example, CMYK, for a printer for a proofer image to be outputted, with a color of the proof image to be actually printed out in accordance with the color data, and image data for printing is converted into image data for a printer in accordance with a combined color conversion definition (a device link profile) of the printing profile and the printer profile, so that the proof image is created in accordance with the image data for a printer thus converted. In this manner, it is possible to create a proof image greatly close to the actual printed matter in color.
In order to obtain an image matched in color in accordance with the manner as mentioned above, there is a need to exactly determine the color conversion definition (the input profile) of the input device such as a color scanner and an electronic still camera, and the color conversion definition (the output profile) of the output device such as a printing machine and a printer.
When the color conversion definition (the profile) is determined, for example, in case of an input device, the input de-vice is used to read a color chart in which color patches are arranged, so as to convert it into image data, and coordinates (CMYK values or RGB values) of a color space (a device-dependence color space: for example, a CMYK color space consisting of four colors of C, M, Y and K, or an RGB color space consisting of three colors of R (red), C (green) and B (blue)) on the image data are determined. And in addition, a spectrocolorimeter is used to measure the color patches constituting the color chart, so that colorimetric values (L*a*b* values or XYZ values) of a colorimetric color space (a device non-dependence color space: for example, an L*a*b* color space or an XYZ color space) are determined. And thus, the coordinates on the device-dependence color space are associated with the calorimetric values on the device non-dependence color space.
The color conversion definition (the output profile) of the output device is determined in such a manner that image data corresponding to a color chart in which color patches associated with a plurality of sorts of color data sequentially varied in dot %, for example, such as 0%, 10%, . . . , 100%, on each of four colors of CMYK, are arranged, is created; the output device outputs the color chart in accordance with the image data; a spectrocolorimeter is used to measure the color patches constituting the color chart thus outputted; and coordinates of a color space (the device-dependence color space) on the image data thus obtained are associated with the colorimetric values of a calorimetric color space (the device non-dependence color space).
The color conversion definitions (the profiles) can be basically determined in the manner as mentioned above. However, the number of color patches constituting a color chart is not so many in such an extent that it corresponds to the number of color conversion definitions (the profiles), and thus the above-mentioned association is associated with thin coordinates which are considerably rough on a color space basis. This is too rough as the color conversion definitions (the profiles). Accordingly, finally, a desired color conversion definition (a profile) is created in such a manner that a number of pairs of association between the coordinates of a color space (the device-dependence color space) on the image data and the calorimetric values of a calorimetric color space (the device non-dependence color space) is increased by interpolation arithmetic operation and the like in accordance with the association between the coordinates of a color space (the device-dependence color space) on the image data and the calorimetric values of a colorimetric color space (the device non-dependence color space), which is obtained by colorimetry of the color patches constituting the color chart in the manner as mentioned above.
However, even if the color conversion definitions (the profiles) for the input device and the output device are determined in the manner as mentioned above, a measurement error by a spectrocolorimeter, a color error of a color patch due to the property (for example, a position dependency of an output color) of a device of interest as an object intended to create the associated color conversion definition (the profile), and adding color patches of a specified important color such as a skin color as color patches constituting a color chart in order to enhance practical color matching accuracy, will cause a distortion on the created color conversion definition (the profile). This will bring about noises destroying monotonicity of variations of L*a*b* data and XYZ data associated with the monotonous coordinate variation on the device-dependence color space.
FIG. 11 is a view showing an example of a color conversion definition (a TARGET device profile) including noises destroying monotonicity of variations of data. The horizontal axis denotes dot % for M (gradation in which a density of a color continuously varies from Y to R) to 100% of dot % for Y. The vertical axis denotes variations of L*a*b* values resulting from variations of dot % for M.
According to the example shown in FIG. 11, the monotonicity of variations of L*a*b* values to the monotonic variations of dot % for M is destroyed by noises included in the color conversion definition (target device profile), so that a slope is greatly varied.
FIG. 12 is a view showing an example of a color conversion definition (a device link profile) including noises destroying monotonicity of variations of data. Here, let us take notice of C and M. The horizontal axis denotes dot % for an input device (gradation in which a density of a color continuously varies from C to M). The vertical axis denotes variation quantity of dot % for an output device to variations of dot % for the input device.
According to the example shown in FIG. 12, there is varied the variation quantity of dot % for the output device, owing to noises included in the color conversion definition (the device link profile), to the monotonic variations of dot % for the input device in which dot % for C is monotonically varied from 100% to 0%, and dot % for M is monotonically varied from 0% to 100%.
Inclusion of the noises as shown in FIG. 11 and FIG. 12 in the created color conversion definition (the profile) would cause discontinuous variations in density of a color (hereinafter, the discontinuous variations in the image density will be referred to as a tone jump), in the event that this color conversion definition (the profile) is used to perform a color conversion, on a portion of gradation in which density of a color is continuously varied, on a color image outputted in accordance with image data after color conversion. And thus the tone jump is visually recognized. This causes a possibility that image quality of the outputted color image is degraded. Further, when it is intended to determine through a computation a reverse conversion profile for determining coordinates of a color space (a device-dependence color space) on image data from calorimetric values of a calorimetric color space (a device non-dependence color space) in accordance with the created color conversion definition (the profile), there is a possibility that the reverse conversion profile cannot be determined exactly because noises included in the color conversion definition (the profile) become a computing error main cause.
It is known that an application of a smoothing processing to a color conversion definition (a profile) including noises destroying such monotonicity makes it possible to ease distortion of the color conversion definition.
FIG. 13 is a view showing an example of a color conversion definition (a target device profile) after the smoothing processing to the color conversion definition (the target device profile) shown in FIG. 11. In a similar fashion to that of FIG. 11, the horizontal axis denotes dot % for M (gradation in which a density of a color continuously varies from Y to R) to 100% of dot % for Y. The vertical axis denotes variations of L*a*b* values resulting from variations of dot % for M.
According to the example shown in FIG. 13, it is eased that a slope of L*A*a*b* values is greatly varied to a monotonic variation of dot % for M.
FIG. 14 is a view showing an example of a color conversion definition (a device link profile) after the smoothing processing to the color conversion definition (the device link profile) shown in FIG. 12. In a similar fashion to that of FIG. 12, here, let us take notice of C and M. The horizontal axis denotes dot % for an input device (gradation in which a density of a color continuously varies from C to M). The vertical axis denotes variation quantity of dot % for an output device to variations of dot % for the input device.
According to the example shown in FIG. 14, there is eased dispersion of variation quantity of dot % for an output device to monotonic variations of dot % for the input device.
When the color conversion definitions (the profiles) after the smoothing processing, as shown in FIGS. 13 and 14, are used to perform a color conversion, the tone jump is eased so that a smooth gradation is expressed, and thus it is possible to output a color image having a high color quality.
However, an application of the smoothing processing to the profile causes data of the profile to vary. And thus there is a possibility that accuracy of the color matching is decreased.